Semiconductor switches used in radio frequency (RF) applications, such as GaAs field effect transistors, PIN diodes, etc., exhibit high insertion losses (e.g., 1-2 dB) and poor isolation in their off-states due to parasitic effects. MEMS switches, on the other hand, can replace conventional resonators, filters and semiconductor switches within many of these RF applications, such as satellite and wireless communications systems, commercial and military radar, global positioning systems and instrumentation systems. Compared to conventional semiconductor devices, MEMS switches have improved insertion losses (e.g., 0.1 dB up to 40 GHz) and very high isolation (e.g., >27 dB) when in their off-states, and typically consume negligible power during each switching cycle.
Signal switching within a MEMS device is accomplished by mechanical deflection of a suspended structure, such as a cantilever, which produces a metal-to-metal contact or, alternatively, capacitive coupling. While several actuation methods have been developed, such as electrostatic, electromagnetic, thermal and piezoelectric, the majority of MEMS switches are electrostatic. Unfortunately, electrostatic actuation is a non-linear mechanism that induces high acceleration levels at switch closure, resulting in heavy switch contact pounding. Further, electrostatic actuation requires high operating voltages at higher switch frequencies. For example, switching times below 5 μs typically requires greater than 50 V. Piezoelectric actuation offers many advantages over electrostatic, electromagnetic and thermal actuation methods, such as improved control, reliability, switching times, isolation, etc.
Here, piezoelectric actuation is based on the converse piezoelectric effect, in which a piezoelectric material produces a strain under the influence of an electric field. These materials mechanically expand and contract in response to an applied voltage, and, unlike traditional electrostatic MEMS switches, the closing force developed by a piezoelectric switch can be significantly improved by increasing the bias voltage (electric field strength) across the piezoelectric material. Additionally, the energy density available in ferroelectric thin films, such as PZT (lead zirconium titanate), is much greater than electrostatic materials, which advantageously increases the potential for conversion to mechanical work. However, MEMS piezoelectric switches incorporating ferroelectric thin films are difficult to produce, the residual stresses within the layers of the switch are difficult to control and the direction of switch flexure is often opposite to the desired effect.